1. Field of the Invention
The present invention relates to an electrostatic actuator using an electrostatic force for displacing opposing electrode members relative to one another, wherein the electrostatic force is generated by a drive power source which applies a voltage between the opposing electrodes. The invention also relates to a method for manufacturing such an electrostatic actuator. More specifically, the present invention relates to a method for forming a hydrophobic film on the surface of at least one of the two electrode members of the electrostatic actuator.
2. Description of the Related Art
Actuators with a microstructure formed using semiconductor microprocessing technologies are widely used in ink jet heads for ink jet printers. These microstructure actuators can be driven in various ways, one of which is electrostatic drive, a method that uses electrostatic force for drive power. Examples of ink jet heads that use electrostatic force to eject ink drops may be found in JP-A-5-50601 (1993), 6-71882 (1994) and EP-A-0 580 283.
This type of ink jet head has, in communication with each nozzle, a respective ink chamber whose bottom is formed as an elastically deformable diaphragm. The diaphragm is disposed opposite a substrate with a certain gap therebetween. Mutually opposing electrodes are also disposed on or by the diaphragm and substrate, respectively, and the space between the electrodes is sealed. In this case, the diaphragm and the substrate form the two opposing electrode members of the electrostatic actuator. When a voltage is applied to the electrodes, the electrostatic force created in the gap causes the bottom of the ink chamber, i.e., the diaphragm, to vibrate as a result of the electrostatic attraction to and repulsion from the substrate. The change in the internal pressure of the ink chamber resulting from this vibration of the ink chamber bottom causes one or more ink drops to be ejected from the ink nozzle. A so-called "ink-on-demand" drive method whereby ink drops are ejected only when needed for recording can thus be achieved by controlling the voltage applied to the electrodes of the electrostatic actuator.
If moisture gets on the opposing surfaces of the opposing electrodes (i.e., on the bottom surface of the ink chamber and on the opposing surface of the opposing substrate) while the ink jet head is being driven by repeatedly applying a voltage to the electrodes, the charge of polar molecules may cause a drop in electrostatic attraction or repulsion properties. If polar molecules adhering to the opposing surfaces form hydrogen bonds, the bottom of the ink chamber (i.e., the diaphragm) may stick to the substrate and can become inoperable.
One possibility of avoiding these problems is to treat the opposing surfaces so that they are made hydrophobic. One means of achieving this is to coat these surfaces with an oriented monolayer of perfluordecanoic acid (PFDA).
An electrostatic actuator which is used for moving micro mirrors and in which PFDA is used for hydrophobic treatment is proposed, for example, in JP-A-7-13007 (1995) and in corresponding U.S. Pat. No. 5,331,454. These documents are directed to a method of preventing the opposing electrode surfaces of the actuator from sticking together when driven by forming an oriented monolayer of PFDA on these surfaces.
Hydrophobic processing using PFDA, however, leaves the following problems to be solved. First, the durability of the PFDA layers formed by simply depositing PFDA on the opposing surfaces of electrode members displaceable relative to one another is insufficient. Consequently, the PFDA layer separates from the surface of the underlying electrode members as a result of the electrostatic field being repeatedly generated between the electrode members to repeatedly displace them relative to each other. These separated layer particles then tend to clump together, creating foreign matter inhibiting relative displacement between the electrode members. When such foreign matter is formed, the danger of the electrostatic actuator becoming inoperable arises.
The gap between opposing electrode members in an electrostatic actuator is preferably as narrow as possible in order to generate a sufficiently high electrostatic force at a relatively low voltage. It is also preferable to minimize this gap as much as possible in order to reduce the size and to achieve a higher density arrangement of electrostatic actuators. PFDA molecules are relatively large, however, and if the gap becomes too narrow, it is not possible to deposit PFDA on the opposing surfaces separated by this narrow gap.
It has also been proposed to use a hexamethyldisilazane (HMDS) film to prevent relatively movable members in a microstructure from sticking together. However, such proposal does not provide any suggestion of sealing a gap in an electrostatic actuator using an HMDS film in the manner proposed by the present inventors.
Therefore, it is an object of the present invention to overcome the aforementioned problems. It is another object of the present invention to provide an electrostatic actuator having a durable hydrophobic film, and to provide a manufacturing method therefor. It is yet a further object of the present invention to provide an electrostatic actuator comprising a hydrophobic film that can be deposited on the surfaces of opposing electrode members, which are displaceable relative to each other by an electrostatic force, even when the gap between the opposing electrode members is narrow, and to provide a manufacturing method therefor.